The invention relates generally to processes for electrostatic image development in toning systems that employ a two-component developer. More specifically, the invention relates to apparatus and methods for electrostatic image development, wherein the image development process is optimized by manipulating certain relationships between carrier particle size, toner particle size, carrier dielectric constant or conductivity, and toner charge to minimize attractive forces between the toner particles and carrier particles that arise from the effects of particle polarization and non-uniform surface charge distributions.
Processes for developing electrostatic images using dry toner are well known in the art. Such development systems are used in many electrophotographic printers and copiers (collectively referred to herein as “electrophotographic printers” or “printers”) and typically employ a developer consisting of toner particles, hard magnetic carrier particles and other components. In many current and prior art developers, the carrier particles arc much larger than the toner particles, on the order of up to 30 times larger.
The developer is moved into proximity with an electrostatic image carried on a photoconductor, whereupon the toner component of the developer is transferred to the photoconductor, prior to being transferred to a sheet of paper to create the final image. Developer is moved into proximity with the photoconductor by a rotating toning shell, an electrically-biased, conductive metal roller that is rotated cocurrent with the photoconductor, such that the opposing surfaces of the photoconductor and toning shell travel in the same direction. Located inside the toning shell is a multipole magnetic core, having a plurality of magnets, that is either fixed relative to the toning shell or that rotates, usually in the opposite direction of the toning shell. The developer is deposited on the toning shell and the toning shell rotates the developer into proximity with the photoconductor, at a location where the photoconductor and the toning shell are in closest proximity, referred to as the “toning nip.”
On the toning shell, the magnetic carrier component of the developer forms a “nap,” similar in appearance to the nap of a fabric, because the magnetic particles form chains of particles that rise vertically from the surface of the toning shell in the direction of the magnetic field. The nap height is maximum when the magnetic field from either a north or south pole is perpendicular to the toning shell. Adjacent magnets in the magnetic core have opposite polarity and, therefore, as the magnetic core rotates, the magnetic field also rotates from perpendicular to the toning shell to parallel to the toning shell. When the magnetic field is parallel to the toning shell, the chains collapse onto the surface of the toning shell and, as the magnetic field again rotates toward perpendicular to the toning shell, the chains also rotate toward perpendicular again. Thus, the carrier chains appear to flip end over end and “walk” on the surface of the toning shell and, when the magnetic core rotates in the opposite direction of the toning shell, the chains walk in the direction of photoconductor travel.
The toner component of the developer is carried along with the carrier particles by virtue of the attractive forces that cause the toner particles to bind to the carrier particles. These forces include surface forces, or adhesion forces, such as van der Waals forces, and electrostatic forces arising from both free charges, such as tribocharge, and bound charges due to polarization induced by those charges and polarization of the particles by the external electric field of image development. Surface forces are important for small toner particles but are generally of very short range and are only significant for particles in contact. However, tribocharging can produce patches of charge at the point of contact between particles, causing uneven charge distribution that can result in a very large attractive force between particles.
While these attractive forces are required to transport toner into the toning nip, image development cannot occur unless the toner particles are separated from the carrier particles. Accordingly, it is important for optimal image development to strike an appropriate balance, such that the attractive forces between the toner and carrier particles are strong enough to efficiently transport toner while at the same time the attractive forces should not be so strong as to interfere with stripping of toner particles from the developer in the presence of the force due to the imaging field, or toner development will be impaired. Accordingly, there is a need in the art for developer and developer systems that strike the appropriate balance by minimizing unwanted components of the attractive forces between toner and carrier particles, allowing for optimal toning efficiency.